Friday, January 15, 2016

Living On Radiation, Fungal Adaptation in The Chernobyl Reactor

by MWIn 1991 Russian scientists published an article inspired by the observation of fungal growth within nuclear reactor no. 4 in Chernobyl, the site of the tragic meltdown in 1986. [1] Which irradiated the surrounding town making it unsuitable for human life. Since then, the site had been a hotbed of biological research into the effects of prolonged radiation on living organisms. The fungal growth that the researchers observed was within the reactor itself, the point with the highest level of radiation. The fungi appeared to be thriving in the unlikely environment; the radioactive water that was used to cool the reactor was blackened with growth. This rose questions into the radiodurability of these fungi, and how they were able to thrive is such a strange place. The authors of the article were forthcoming with a theory; that the fungi were using the radiation as an energy source, the same way some bacteria and all plants use sunlight. However, in this instance the authors hypothesized the fungi were accomplishing it with the pigment melanin as their chlorophyll. This was a grand theory, but was equally hard to prove. The effects of radiation on these fungi are still poorly understood.

The idea of radiotrophy is not so far-fetched, but is definitely odd by todays understanding of how most life works. The radiation from a nuclear reactor is extremely harmful to most organisms, given that it easily produces double-stranded breaks in DNA, which causes very damaging mutations. However, life on Earth has been exposed to various sources of radiation throughout history, sometimes for very extended periods of time. It’s not so crazy to believe that at some point, organisms evolved to not only cope with the danger, but also use it to their advantage. First, let’s start with a quick review of what radioactive means. At its most basic level, radiation is the emission of some form of energy; this includes acoustic, electro-magnetic, and particle radiation. The types that come from things we think of as radioactive, like sites of nuclear reactor meltdown or (fission) nuclear weapons, are gamma and neuton radiation. The radiotrophic fungi are theorized to use the gamma radiation as an energy source, and seem resistant to lingering neutron radiation. Another name for gamma radiation is photon radiation, or light. However the type of gamma radiation coming from the reactor in Chernobyl isn’t visible to humans like the visible spectrum of light we are familiar with, its way more energetic and is closer to x-rays, or the cosmic radiation that bombards the atmosphere. Normally, organisms use a variety of chlorophylls to harvest light and convert it to chemical energy. These compounds may blue, green, or several other colors, depending on the wavelengths, or energy level, of light they absorb. The process depends on the excitation of an electron in the pigment structure when a photon hits it, and the complex transfer of the energy to a chemical form. A similar process is theorized to take place with melanin, a dark pigment that appears black in high concentrations but in lower concentrations is what causes the appearance of a tan on human skin after some time is spent in intense sunlight. In most organisms, melanin is theorized to be a protective compound, helping to prevent damage to cells by UV light. However in the radiotrophic fungi, melanin is proposed to function a little like chlorophyll does for plants, as an antenna that gathers electromagnetic radiation allowing its conversion to chemical energy that the cell can use. Melanin is unique however in its ability to absorb much more energetic wavelengths than just visible and UV light.

Three melanin-producing fungal organisms have been directly studied for radiotrophism in response to the Chernobyl observations, one of which was isolated directly from the reactor. Cladosporium cladosporioides, Penicillium roseopurpureum, and Penicillium hirsutum were exposed to gamma-radiation producing particles on a solid medium, and the direction of their growth evaluated. It was shown that even when alternative chemical energy sources were available in a different region of the medium, growth more often developed towards the radioactive source. [2] This however does not prove radiotrophism. The correlation between growth and radioactivity could be due to a number of causes, including any adaptive mechanism to protect the cells from the radiation. Could it be possible the organisms were creating a living shield? As an initial observation, the behavior is still very interesting. In a follow up study, melanized cells fared better than non-melanized cells when exposed to radiation and actually grew faster when exposed to radiation when compared to an identical culture grown without radiation. [2] However, this again does not prove that the melanized cells were deriving energy from gamma radiation. In the comparison between melanized and non-melanized cells exposed to radiation, the melanized cells would be expected to fare better due to the well known protective properties of the pigment. This however does not explain the increased growth of irradiated, melanized cells in comparison to non-irradiated, melanized cells.
To further explore the possible role of melanin in radiotrophy, researchers next inspected the properties of melanin as an energy-collecting molecule. Melanin is unique in its ability to quench free radicals in solution and absorb a wide spectrum of electromagnetic radiation, but its properties are still not fully understood. The researchers showed gamma radiation induced reductive potential in melanin, allowing it to reduce NAD+ to NADH, an electron shuttle used by every living organism that is central to energy metabolism. [3] This suggests a possible mode by which melanin may input energy in response to radiation. But still, a mechanism by which fungi can use melanin to collect energy has not been well characterized or even observed in a living system. The reductive potential of the compound alone is not sufficient to suggest a clear radiotrophy. The possibility of radiotrophism is still exciting, but needs significantly more research to confirm. The current research is somewhat limited, but the implications of radiotrophy are exciting. Not only does it suggest an even wider range of conditions conducive to life, but it would add a powerful tool to bioengineering technology as well. Bioremediation of dangerous radioactive substances is only one of the useful innovations that might be made possible. Ultimately, radiotrophism may be the key to engineering organisms that can survive conditions that have since been inaccessible to bioengineering, such as the upper atmosphere, and the unshielded regions of orbiting space stations.

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This blog represents the work of students from one of the few courses devoted to Eukaryotic Microbiology as a whole. This is a new experiment for this course that we hope will be a successful fusion of student learning and science communication. Enjoy, but play nice, comments are welcome but mean-spirited comments will be deleted.